This invention relates to electret filter media having enhanced charge stability.
Electret filter media have long been used in many filtration applications. Electret filter media are those that include a dielectric insulating polymer web that is treated to possess substantially permanent spatially oriented, opposite charge pairs or dipoles. Among the common polymer webs used for electret filter media are polypropylene, polyethylene, polyester, polyamide, polyvinyl chloride, and polymethyl methylacrylate.
Conventional filter media are substantially lacking in electrostatic charge and rely upon impingement, impaction and diffusion for filter performance. Electret filter materials offer improved filtering performance over conventional filter materials. The presence of oriented dipoles in the electret filter media is believed to enhance filter performance by allowing the filter media to attract and maintain charged and uncharged particles to be filtered.
Electret filter materials are made by a variety of known techniques. One technique for manufacturing electret filter media involves extruding a polymer, typically having a high melt flow index, through a die having a linear array of orifices. An air knife is used to attenuate the extruded polymer fibers by a ratio of about 300:1. The attenuated fibers, having diameters of about one to ten micrometers, are collected on a rotating drum or moving belt using a moderate vacuum. The fiber web is then treated to impart on the fiber web charge pairs or dipoles. The charge pairs or dipoles can be imparted to the fiber, for example, using AC and/or DC corona discharge.
One problem associated with electret filter material is that the charge pairs or dipoles imparted to the filter media often are not stable. In some instances, charge or its spatial orientation is lost after filtering certain contaminants for relatively short time periods. The result is a marked decrease in filter performance over a relatively short period of time (e.g., less than 20 minutes). The National Institute of Safety and Health (NIOSH) has established standards for performance of certain filters. The NIOSH standards evaluate filters in a carrier after a 200 milligram filtration challenge. One challenge, of solid aerosol particles, evaluates filter performance against solid sodium chloride particles suspended in air. Another challenge, of liquid aerosol particles, evaluates filter performance against liquid droplets of dioctyl phthalate (DOP) suspended in air. Electret filter media generally are able to maintain charge stability and filter performance when filtering solid aerosols, including the sodium chloride test standard. However, liquid aerosols tend to degrade the charge on the electret filter media, and thus filter performance diminishes after only a short period of filtration.
There is thus a need for electret filter media having improved charge stability that are able to maintain acceptable filter performance over time.
Accordingly, it is an object of the present invention to provide a charge stabilized electret filter media. Another object of the invention is to provide electret filter media that are able to maintain filtration performance even after prolonged filtration challenges. It is also an object of the invention to provide filter media that have enhanced filtration performance characteristics. A further object of the invention is to provide electret filter media suitable for a variety of filtration applications, including use in industrial face masks. Other objects will be readily apparent to one having ordinary skill in the art upon reading the following description.
The filter media of the present invention is an electret polymeric fiber web that includes a charge stabilizing additive. The charge stabilizing additive may be a fatty acid amide; a nonionic, oleophobic fluorochemical surfactant; or mixtures of such compounds. Typically the charge stabilizing additive is present at a concentration in the range of about 0.1 to 2.0% by weight.
The electret polymer fiber web can be made from a variety of polymeric materials, including polypropylene, polyester, polyamide, polyvinyl chloride, polymethylmethacrylate, and polyethylene. Polypropylene is among the more preferred polymeric materials. Typically, the polymer fibers that form the web have a diameter in the range of about 1 to 20 micrometers.
The electret filter media of this invention is characterized by improved filtration performance and enhanced charge stability of the electret polymer web. In particular, the filter media is able to provide desirable filtration properties, as indicated by alpha value, despite continued filtration challenge.
In another aspect the invention relates to a method of manufacturing an electret filter material. According to this aspect of the invention a polymer resin, having incorporated therein a charge stabilizing additive of the type noted above, is processed to yield a web of melt blown polymer fiber in which the charge stabilizing additive is present at a concentration range of about 0.1 to 3.5% by weight. Subsequently, the web is treated to form permanent dipoles in the melt blown polymer web. The permanent dipoles can be imparted to the web by a variety of techniques including AC and/or DC corona discharge.
In one embodiment the manufacturing process can be modified by heat treating the polymer web, which has incorporated therein a charge stabilizing additive, before charging the material by AC and/or DC corona discharge.
The filter material of the present invention can be used in a variety of filtration applications in which a relatively stable alpha value is desired. Among the uses for such filter media are industrial face masks, ASHRAE filters, HEPA filters, and ULPA filters.
All percentages by weight identified herein are based on the total weight of the web unless otherwise indicated.